In a usual LTE (Long Term Evolution) deployment a number of eNodeBs (Evolved Node B or eNB) are deployed to provide coverage in a specific area. Each eNodeB can manage a set of cells and all UEs (User Equipment) that are in the coverage area of those cells.
From the UE perspective, the cells are distinguished by a physical-layer cell identity (PCI) which is defined in 3GPP TS 36.211 Ch 6.11. In a normal deployment scenario, neighboring cells have different PCIs and when UEs are in a connected state (i.e. not idle) they use these PCIs as an identifier for handover measurement.
Cell Merge, also called shared cell or multi-sector cell in some cases, is a new cell configuration for LTE and enables a multi Radio Resource Unit (RRU) deployment that is not dependent on cell planning from a Radio Frequency (RF) perspective. It is achieved by allowing different RRUs to use the same PCI. As a result, all RRUs are considered by the UE to be part of the same cell. The spatially separated RRU or a group of RRUs are called a sector. A cell can contain multiple sectors, and a UE can belong to one sector or multiple sectors depending on the degree of sector isolation.
In the basic LTE cell configuration, all of the UE's (User Equipment) camped in that cell shall share cell resources by time and/or frequency multiplexing. In a multi-sector cell, yet another resource domain, a spatial resource is introduced. UEs share cell resources also by SDM (Space-Division Multiplexing).
FIG. 1 shows UE signal detection by different cell sectors in accordance with SDM. A single UE, UE1 can be within radio contact with three different sectors S1, S2, S3 of a single cell. A first radio signal path P1 connects to a macro sector S1 antenna array A1. A second path P2 connects to a pico sector S2 within the macro sector and a third path P3 connects to a second pico sector S3. The first UE UE1 is located between the two pico sectors and within the macro sector of a single cell. A second UE, UE2, farther away from the pico sectors communicates with the macro sector but may still be able to interfere with one or both of the pico sectors even if it is beyond the range of the pico sector antennas. This is inter-sector interference (ISI).
Some UE's that are spatially separated can use the same time and frequency resource, but on different sectors. In some cases, antenna feeders and antenna placement cannot be altered or planned, leading to ISI, where a UE's transmission can be detected in several sectors. In uplink (UL), the received signals detected in the multiple antennas can be selected to combine in order to obtain macro diversity gain; in downlink (DL), the signal is only transmitted in the selected antennas to enable higher transmission energy.
There are several benefits for this configuration. A first benefit is easy cell planning. All sectors belong to the same cell, so there is no need to consider inter-cell interference. A second benefit is reduced L3 (Layer 3) control signaling, because there is no need to perform handover between sectors within one cell. A third benefit is that, for the UEs belonging to multiple sectors, macro diversity gain can be achieved in the uplink by selectively combining the signals from multiple sectors. In the downlink, eNodeB may selectively transmit in one or multiple sectors. The UE can combine the received signals from transmitted sectors. For the UE belonging to sectors which are spatially isolated, multiplexing different UEs in the same time and frequency resources can be used to improve capacity.
FIG. 2 shows the Handover procedure in LTE between eNodeBs without a MME (Mobility Management Entity)/serving GW (Gateway) change as defined in 3GPP TS 36.300. After the source eNodeB receives the measurement reports 2 from the UE, the source eNodeB decides to trigger the handover 3 by starting to prepare the target cells.
Focusing on the handover preparation phase, the source eNodeB issues a Handover Request message 4 to the target eNodeB(s) passing all necessary information to prepare the Handover on the target side. The RRC (Radio Resource Control) context included in the Handover Request message contains all the current UE configurations in the source cell. Admission Control 5 may be performed by the target eNodeB after receiving the Handover Request if the resources can be granted by the target eNodeB. Then the target eNodeB configures the required resources in the target cell and sends a Handover Request Acknowledge 6 to the source eNodeB. The Handover Request Acknowledge message includes a transparent container to be sent to the UE as an RRC message (i.e. RRC Connection Reconfiguration) 7 to perform the handover.
For improved efficiency and user experience, handover decisions and inter-cell interference management may be improved to accommodate multi-sector cell deployments.